3.5 Analytical Chemical Standards
119
(8) Primary analytical standards should be easy to prepare (condition) for use
and also to store.
(9) Secondary analytical standards should provide a clear, unequivocal traceability pathway to primary standards. Once prepared and checked, they
should exhibit adequate, accurately known stability for a fIxed period of
time at the least.
3.S.3 Preparation and Storage
These two operations are crucial with a view to ensuring proper handling and
usage of primary analytical standards. Obviously, the more complicated these
operations are the more expensive to conduct they will be and the wider will be
their sources of variability - and, also, obviously, the higher will be the uncertainty in the reference quantities. The specifIc preparation and storage
procedures to be used vary with the nature of the particular standard.
The physical standards used to calibrate instruments are prepared by commercial manufacturers or competent bodies. They require little conditioning
prior to use (e. g. transfer weights need only be cleaned with ethyl ether to remove
any dust or grease accidentally left by the operator). Each standard should be
stored as per the exact instructions usually supplied by the manufacturer.
Because primary standards are pure or mixed substances, their preparation
for use involves procedures of variable complexity dependent on their state of
aggregation and the properties that are to be used as references. Gaseous
standards are especially complex. At present, they are supplied in permeation
tubes impregnated with them; on insertion into specially designed assemblies,
the tubes release a given amount of S02, NOn etc., into a carrier gas that is
circulated at a flow-rate carefully adjusted through temperature. Volatile substances used as. standards (e.g. pure and mixed aliphatic hydrocarbons for calibratinggaschromatographs) are usually available in sealed vials. Solid standards
(e.g. titrimetric standards such as CaC03, potassium hydrogen phthalate,
Na2C03, KID3) usually require drying at a given temperature over a preset time to
remove moisture; they are usually stored in weighing bottles that are placed in
containers. the atmosphere in which is kept dry by means of a desiccating substance (e.g. P20S). This type of standard is usually prepared by a manufacturer;
its price is proportional to the complexity of the operations involved - the purer
the standard, the higher the price. The homogeneity and stability of CRMs are
assured by competent bodies and only after thorough testing.
__ Box3.l1
A result of a CMP conducted by a routine laboratory can be made traceable in various ways.
First, one must decide which will the last link in the comparison chain be. Whenever
possible, it should be an 51 unit (the kilogram or mole); however, this is usually impossible.
Alternatively, one can use a substitute reference such as a CRM or a primary method. The
important thing is that this new reference or standard be accurately specified so that results
can be reliably compared.
119
(8) Primary analytical standards should be easy to prepare (condition) for use
and also to store.
(9) Secondary analytical standards should provide a clear, unequivocal traceability pathway to primary standards. Once prepared and checked, they
should exhibit adequate, accurately known stability for a fIxed period of
time at the least.
3.S.3 Preparation and Storage
These two operations are crucial with a view to ensuring proper handling and
usage of primary analytical standards. Obviously, the more complicated these
operations are the more expensive to conduct they will be and the wider will be
their sources of variability - and, also, obviously, the higher will be the uncertainty in the reference quantities. The specifIc preparation and storage
procedures to be used vary with the nature of the particular standard.
The physical standards used to calibrate instruments are prepared by commercial manufacturers or competent bodies. They require little conditioning
prior to use (e. g. transfer weights need only be cleaned with ethyl ether to remove
any dust or grease accidentally left by the operator). Each standard should be
stored as per the exact instructions usually supplied by the manufacturer.
Because primary standards are pure or mixed substances, their preparation
for use involves procedures of variable complexity dependent on their state of
aggregation and the properties that are to be used as references. Gaseous
standards are especially complex. At present, they are supplied in permeation
tubes impregnated with them; on insertion into specially designed assemblies,
the tubes release a given amount of S02, NOn etc., into a carrier gas that is
circulated at a flow-rate carefully adjusted through temperature. Volatile substances used as. standards (e.g. pure and mixed aliphatic hydrocarbons for calibratinggaschromatographs) are usually available in sealed vials. Solid standards
(e.g. titrimetric standards such as CaC03, potassium hydrogen phthalate,
Na2C03, KID3) usually require drying at a given temperature over a preset time to
remove moisture; they are usually stored in weighing bottles that are placed in
containers. the atmosphere in which is kept dry by means of a desiccating substance (e.g. P20S). This type of standard is usually prepared by a manufacturer;
its price is proportional to the complexity of the operations involved - the purer
the standard, the higher the price. The homogeneity and stability of CRMs are
assured by competent bodies and only after thorough testing.
__ Box3.l1
A result of a CMP conducted by a routine laboratory can be made traceable in various ways.
First, one must decide which will the last link in the comparison chain be. Whenever
possible, it should be an 51 unit (the kilogram or mole); however, this is usually impossible.
Alternatively, one can use a substitute reference such as a CRM or a primary method. The
important thing is that this new reference or standard be accurately specified so that results
can be reliably compared.